Braking systems for towed vehicles, such as horse trailers and the like for example, are often provided for braking the wheels of the towed vehicle to improve stopping distance and/or stability of the towing/towed vehicle combination. While various types of trailer braking systems exist, including electric, surge, and vacuum over hydraulic systems, one type of system that has been particularly popular is the electro-hydraulic type system.
A typical electro-hydraulic braking system includes conventional hydraulic brakes for braking the wheels of the towed vehicle connected to an electro-hydraulic pressure source that supplies pressurized fluid to actuate the brakes. The supply of pressurized fluid to the hydraulic brakes is typically modulated in response to an electrical signal generated by the towing vehicle during application of the towing vehicle's brakes. The pressure supplied to the hydraulic trailer brakes, and thus the amount of braking force generated by the hydraulic trailer brakes, can be automatically modulated in proportion to the level of braking of the vehicle, or in any other desired manner.
In the past, electro-hydraulic pressure sources have included a hydraulic pump mounted to the towed vehicle and driven by an electric motor supplied with electricity from the tow vehicle. An external reservoir is typically mounted to the towed vehicle for supplying hydraulic fluid to the pump.
In a typical system, two electrical wires carry current from the towing vehicle to the towed vehicle via any of a variety of conventional electrical connection devices. The first wire carrying electric current to the towed vehicle is connected to the electric motor. The second wire, which carries the generated signal, is connected to a device for controlling the pressure supplied to the hydraulic brakes of the towed vehicle, such as a variable valve mechanism (e.g., a proportional valve). As will be appreciated, the towed vehicle can also have a separate power source, such as a battery, and a breakaway device for applying the trailer brakes in the event the towed vehicle separates from the towing vehicle.
When electric current is supplied to the first electric wire, the electric motor drives the hydraulic pump which in turn is interconnected hydraulically with hydraulic fluid lines leading to conventional hydraulic brake cylinders located in each wheel of the towed vehicle. The aforementioned device for controlling the pressure supplied to the hydraulic brakes of the towed vehicle is interconnected hydraulically between the outlet of the hydraulic pump and the inlet thereof, in a manner such that hydraulic fluid flowing out of the outlet of the hydraulic pump and to the hydraulic wheel cylinders must flow through said variable valve means before it can return to the inlet side (e.g., the reservoir) of the hydraulic pump.
Thus, upon application of electrical current to the direct current electric motor via the electrical connection between the two vehicles, the hydraulic pump begins to circulate fluid which flows from the outlet of the pump to provide pressure to the hydraulic brake cylinders located in each wheel of the towed vehicle. Pumped fluid returns to the inlet side of the pump via a variable valve means.
Upon application of electric current to an electromagnet of the variable valve mechanism, flow through the valve is restricted incrementally in response to the amount of electric current supplied to the electromagnet. As flow through the variable valve mechanism becomes so restricted, there will be a resulting increase in hydraulic pressure supplied to the wheel cylinders of the towed vehicle, because hydraulic fluid, being pumped at a substantially constant rate by the hydraulic pump, will not be allowed to return to the inlet side of the pump at the same rate at which said hydraulic fluid is being supplied to the hydraulic lines leading to the wheel cylinders. Thus, by supplying electric current to the electric motor at the same instant at which the brakes of the towing vehicle are applied a slight pressure can be supplied to the hydraulic wheel cylinders of the towed vehicle, this initial pressure being controlled by the relationship of the pumping rate of the hydraulic pumping means to an orifice size of the variable valve in its completely open position. When further braking pressure is applied in the towing vehicle, variable electric current is supplied to the electromagnet in the variable valve causing a further restriction in the circulation of hydraulic fluid through the system, with the resulting increase in braking pressure supplied to the hydraulic wheel cylinders of the towed vehicle which is proportional to the amount of electric current supplied to the electromagnet of the variable valve.